1. Field of the Invention
The invention relates to a method of forming a solid body using a two-dimensional image printer, wherein curable liquids of different colors are used for printing and curing droplets in several layers one upon the other, and colors of the liquids for forming a surface layer of the body are selected from a specific set of surface colors.
2. Description of the Related Art
Methods are known for elevated printing or 3D printing using a two-dimensional image printer. For a relatively flat body, this is also known as relief printing, or 2.5D printing. The two-dimensional image printer may, for example, be an ink jet printer, using UV-curable inks as the liquids which, after printing and curing with UV-radiation, will form the solid body. Alternatively, an image printer may apply phase-change inks, that are liquid at an elevated temperature and solidify by cooling down.
When a body is intended to have a specific color or color pattern on its surface, it is sufficient to use the corresponding surface colors only for printing a surface layer of the body, whereas inks in other colors may be used for forming the interior of the body. This permits to increase the efficiency of the elevated printing process. For example, when the printer has a plurality of print heads for different colors, all print heads may be used simultaneously for quickly printing the bulk of the body, whereas the surface layer is printed only with the print heads that contain the liquids in the appropriate surface colors. Moreover, when the ink used for the interior of the body is different from the ink used for the surface, it is possible to save costs by using cheaper inks for the interior of the body and/or to improve the physical properties of the body by selecting inks with appropriate properties for the interior of the body.
In patent application US2002/0167101 an apparatus is disclosed wherein a first material is used in molding the interior of a layer of a 3D molded product and a second material is jetted in molding a surface of the layer. The second material includes a plurality of resins having different color components. The first material may be an uncolored resin as the interior material is invisible to the outside.
In patent application US2005/0046684 photo-curable transparent ink droplets are applied in addition to photo-curable color ink droplets in order to be able to apply a uniform total amount of ink on a recording medium in various structures.
The color impression of the surface layer will depend not only on the selected inks but also on the thickness of this layer, because the light will penetrate to a certain depth into the interior of the body before it is reflected, so that the absorption of the material closely underneath the surface will also influence the spectrum of the reflected light. For example, when white ink is used for forming a white primer layer near the surface of the body, and a surface color is used for printing a surface layer on the white background, this color will appear bright and brilliant when the surface layer is thin, and it will appear darker and become more saturated when the thickness of the surface layer is increased.
The printer applies the ink droplets by jetting them in a certain jetting direction onto the substrate on which the body is to be formed and onto ink layers that have been formed already, respectively. For the purposes of this disclosure, the jetting direction shall be assumed to be the vertical direction.
When the droplets are jetted out in the liquid state and impinge on the substrate or a lower layer of the body, the ink will spread to some extent in the direction normal to the jetting direction before it is cured. Consequently, each droplet will form, after curing, a lens-shaped ink dot with a relatively large diameter, e.g. 60 μm (corresponding to the size of one pixel in conventional two-dimensional printing) in the directions normal to the jetting direction, whereas the dimension in the jetting direction is significantly smaller (e.g. 1-10 μm). Consequently, when the surface layer is formed by only a single layer of droplets, the thickness will be small at the top surface, where the surface layer is flat and extends essentially at right angles to the jetting direction, whereas the thickness of the surface layer will be significantly larger (in the order of magnitude of the above-mentioned relatively large diameter) in surface regions where the surface the body forms a steep slope or extends even vertically. As a consequence, the color impression will not be as uniform as desired.
Moreover, when the thickness of the surface layer corresponds to only a single droplet in essentially vertical surface regions, the “pile” of cured droplets that form the surface layer may become instable and irregular, due to inevitable variations in the positions at which the droplets are deposited. This effect may lead to a rough and non-uniform appearance of the surface.
It is an object of the invention to improve the surface quality of a body formed by elevated printing, in particular the surface color quality.